1. Field of the Invention
The invention relates to a control device for an automatic transmission.
2. Description of Related Art
Generally, various hydraulic actuators are equipped for an automatic transmission that is mounted on a vehicle. For example, a belt-drive continuously variable transmission (CVT) that steplessly changes a speed ratio includes various hydraulic actuators including hydraulic cylinders respectively provided at a movable sheave of a primary pulley and a movable sheave of a secondary pulley.
Such hydraulic actuators are actuated through control over hydraulic control valves that operate using control hydraulic pressures as pilot pressures. The control hydraulic pressures are respectively supplied from solenoid valves. Each of these solenoid valves adjusts the control hydraulic pressure that is supplied to a corresponding one of the hydraulic actuators on the basis of a signal that is input from an electronic control unit. Thus, a speed ratio, a belt clamping pressure, a line pressure, a lockup engagement pressure, and the like, are adjusted to optimal values as needed.
For example, a belt clamping pressure control valve is connected to the hydraulic cylinder of the secondary pulley. By controlling the line pressure that is supplied to the belt clamping pressure control valve using the control hydraulic pressure output from the corresponding solenoid valve as the pilot pressure, the belt clamping pressure is controlled. Such a belt clamping pressure is desirably controlled to an optimal value such that a belt slip does not occur.
There is known a control device for an automatic transmission, which executes feedback control over a current flowing through a solenoid of a solenoid valve such that an actual line pressure (hereinafter, referred to as actual line pressure) converges to a target line pressure on the basis of a difference between the target line pressure and the actual line pressure detected by a hydraulic pressure sensor in order to optimally control the belt clamping pressure (for example, see Japanese Patent Application Publication No. 2010-270798 (JP 2010-270798 A)).